Digestive Physiology - Anatomy

The mouth is the start of the gastrointestinal tract. It is the site of both mechanical and chemical digestion via chewing, and saliva, respectively. Saliva contains the enzyme amylase that breaks down carbohydrates.

The mouth, stomach, and small intestine all contain enzymes that help chemically digest food. The mouth contains salivary amylase to digest carbohydrates. The stomach contains pepsin to digest proteins. The small intestine contains numerous enzymes from pancreatic secretions that digest carbohydrates, fats, and proteins.

The esophagus serves to transport food from the mouth to the stomach via smooth muscle contractions. No chemical digestion takes place in the esophagus, and no digestive enzymes are secreted into this region.

All enzymes released into the lumen of the small intestine by the pancreas are zymogens. Enterokinase is an enzyme that activates trypsin in the lumen. Once trypsin is activated, it is capable of activating every other enzyme found in the lumen of the small intestine, even other trypsin enzymes.

Bile is produced by the liver and released from the gall bladder into the small intestine. It has the function of emulsifying fat, increasing its surface area by forming micelles. This allows the protein lipase to break down and digest the fat in the small intestine much more quickly.

The pancreas secretes bicarbonate to neutralize the acidity of chyme and pancreatic amylase to aid in the digestion of carbohydrates.

Cholecystokinin causes the gall bladder to contract so that it can releae bile into the small intestine.

Secretin is released in response to acid in the small intestine, and causes a release of pancreatic bicarbonate to neutralize the acid. Pepsin, trypsin, and carboxypeptidase are all involved in chemical digestion of proteins.

Peristalsis is the rhythmic contraction of smooth muscle to propel digested contents through the digestive tract. It is most commonly examined in the esophagus, as the means by which a bolus is transported from the mouth to the stomach.

Segmentation contractions occur in the small intestine as a means of mixing chyme with digestive enzymes, but do not result in propulsion. Churning movements are most common in the stomach, where food particulates are mechanically digested and exposed to hydrochloric acid. Mastication refers to the mechanical digestion of food via crushing and tearing, primarily achieved when food is chewed in the mouth.

15% of protein digestion occurs in the stomach and pepsin is the enzyme responsible. Remember, pepsin has an optimum pH of about 2.0, which is the pH of the stomach. Trypsin, another protease, has an optimum pH of 7.5-8.5, and therefore would not function in the stomach. Rather, it is produced by the pancreas and carries out its enzymatic reaction in the small intestine. Amylase breaks down carbohydrates. Trypsinogen is the inactive form of trypsin, thus it does not directly break down proteins.

The pancreas is responsible for secreting digestive enzymes into the small intestine. These enzymes are responsible for digesting lipids (lipase), proteins (trypsin), nucleic acids (nucleases), and starches (amylase). In addition, the pancreas secretes bicarbonate to help neutralize the acids coming from the stomach. Pepsin's zymogen, pepsinogen is released by chief cells in the stomach, and gets activated by the acid that is also produced by the stomach (by parietal cells).

Amylase is secreted in active form. Pepsin and trypsin are first secreted as zymogens, inactive precursors that must be activated in order to function. Zymogens are produced with peptides blocking their active sites that must first be cleaved to function. Angiotensin is also secreted as a zymogen, angiotensinogen. Zymogens are useful in allowing the body to quickly activate an enzyme/hormone rather than synthesizing it from scratch.

The body compartmentalizes each hormone to help ensure it is it its optimum pH. Try to remember where each enzyme functions to help remember each enzyme's optimum pH. Pancreatic amylase is secreted by the pancreas into the small intestine. Amylase also occurs in the mouth, which is neutral. It's optimum is around 7. Pancreatic lipase is secreted by the pancreas into the small intestine, which is slightly basic because of the release of bicarbonate. Pancreatic lipase functions best at a pH of about 8. Trypsin is also secreted by the pancreas into the small intestine, and has an optimal pH of about 8. Pepsin functions in the stomach, which is extremely acidic. Pepsin's optimum pH is about 2.

Just after eating, the body is busily converting carbon-based life forms to glucose. If the body is about to have an influx of glucose, the the body must produce more insulin to allow for the passage of glucose into its cells. Glycogen, the stored form of glucose, would naturally increase since the person just replenished lost glucose or energy with food and will store it for later use. Glucagon would decrease because this breaks down glycogen. If the body is receiving fresh glucose, it doesn't need to break down its stores of glycogen into glucose. And finally, if the body is breaking down food into glucose, the liver doesn't need to release more freshly broken down (glucose that was previously stored as glycogen) glucose.

Vitamin B12 absorption requires intrinsic factor (IF), which is a protein secreted by the parietal cells of the stomach. When vitamin B12 binds to IF, it is able to be absorbed by the body in the terminal ileum. Therefore, removal of the ileum would likely result in a vitamin B12 deficiency, since the body cannot absorb the B12-IF complex.

The stomach has the lowest pH, roughly around 2.0 A low pH indicates a high acidic content. The stomach is the most acidic environment in the digestive system as it is the home to parietal cells. Parietal cells secrete hydrochloric acid in response to eating in order to stimulate the acidic breakdown of food.

Salivary amylase is produced by the salivary glands and functions to hydrolyze starch to maltose. Pepsin is produced by the stomach to hydrolyze peptide bonds. Lipase is produced by the pancreas to hydrolyze lipids. Lactase is produced in intestinal glands to hydrolyze lactose to glucose and galactose.

Pepsinogen is the inactivated form of pepsin. Pepsinogen gets activated by an increase in ions (acid), which is released by the parietal cells. Pepsin helps with protein digestion and is released from the chief cells in the stomach.

The tissues in the stomach are also made up of primarily protein. If it was produced in the stomach as its active form, then pepsin could also digest the stomach as well.

The large intestine is responsible for the absorption of water and electrolytes. Most digestion and nutrient absorption occurs in the small intestine, leaving water and insoluble wastes for the large intestine. Once absorption takes place, the wastes become more solid and the body is able to retain water. Loss of this large intestine function can lead to severe dehydration.

Fat digestion is primarily a function of lipase and bile. Lipase helps cleave lipid molecules and bile helps emulsify particles for transport into the lymphatic system via lacteals.

The digestive system is responsible for the breakdown of food as it moves from the mouth, down through the stomach and the intestines. The food is mechanically broken down into small pieces in the mouth, and then further digested by the gastric secretions and contractions of the stomach. The small intestine is the site of chemical digestion due to the presence of pancreatic secretions. Once digested, the nutrients are absorbed in the jejunum and ileum of the small intestine. The large intestine is responsible for absorption of water and electrolytes, after which the remaining products are carried to the rectum for elimination.

If the water is not absorbed due to an infection or anatomical abnormality the feces will retain the water content, resulting in diarrhea.

The kidneys are responsible for the excretion of fluid wastes from the blood and help to maintain water balance and blood pressure. They are not involved in the digestive tract.

Amino acids, sugars, and fats are absorbed in the small intestine, but water is absorbed in the large intestine.

Saliva contains the enzyme alpha amylase. This enzyme is responsible for breaking down starch and carbohydrates into smaller polysaccharide chains.

Later, in the small intestine, pancreatic beta amylase further digests the carbohydrates until they can be absorbed into the blood. Most chemical digestion occurs in the small intestine, where enzymes to digest proteins, lipids, and carbohydrates are introduced to the digestive tract.